As an electric storage device, nonaqueous electrolyte secondary battery cells such as a lithium ion secondary battery cell are conventionally known. Further, as such a nonaqueous electrolyte secondary battery cell, a battery cell including an electrode group as an electrode assembly and a case housing the electrode group that includes electrode plates respectively having positive and negative polarities is known in general.
Specifically, the electrode group has a positive electrode plate and a negative electrode plate as the electrode plates, the positive electrode plate includes a positive electrode current collector and a positive electrode active material supported on the positive electrode current collector, and the negative electrode plate includes a negative electrode current collector and a negative electrode active material supported on the negative electrode current collector. Further, the electrode group is formed by layering the positive electrode plate, the negative electrode plate, and a separator interposed between the positive electrode plate and the negative electrode plate together. Furthermore, an electrolyte solution is injected in the case.
As a known electrode plate, there can be mentioned, for example, those formed by compressing, in the thickness direction, a stack of a current collector and a mixture layer containing a binder and secondary particles each having a hollow region formed therein by aggregation of a plurality of primary particles (fine particles of several μm) of an active material (see JP 2009-283354 A and JP 2011-119092 A).
The secondary particles with such a hollow structure contained in the mixture layer enhance the penetration of the electrolyte solution into the hollow regions of the secondary particles. This improves the ion conductance of the mixture layer, thereby allowing an electrode plate with an improved conductivity to be obtained. Further, such compression as mentioned above brings the current collector and the active material in the mixture layer to close contact with each other. As a result, an increase in resistance due to the above-mentioned hollow regions is suppressed.